An electrode for a fuel cell need to have gas diffusibility and permeability, a strength capable of withstanding the handling, flexibility, a strength capable of withstanding the compression applied when they are produced or assembled into a cell, and so forth. Solid polymer electrolyte fuel cells, in particular, need to be small as compared with phosphoric acid fuel cells, and require thin electrodes. In such fuel cells, the electrodes use, as the electrode substrate, mainly a carbon fiber paper or a carbon fiber fabric. In the carbon fiber paper, in particular, the dispersion of carbon fibers is important and, when the paper is formed in a state that the carbon fibers are not spread from each other and form a bundle, there have been various problems, for example, the non-uniform thickness and non-uniform gas permeability of the paper formed and the difficulty of uniform coating of a catalyst on the electrode substrate produced from the paper.
In, for example, JP-A-11-185771 is disclosed a fiber of 1 to 4 μm in fiber diameter which is convertible into a carbon fiber, or a paper comprising a carbon fiber of 0.5 to 3 μm in fiber diameter. Use of such a small-diameter fiber contributes to a reduction in electrode-inside electrical resistance but makes very difficult the dispersion of fibers when a paper is produced therefrom; therefore, it is difficult to obtain therefrom a carbon fiber paper having carbon fibers uniformly dispersed therein and, when an electrode substrate was produced using the paper, there have arisen the above-mentioned problems such as non-uniform thickness.